Prime editing is another CRISPR based gene-editing technology. Prime editing was developed by David Liu and his lab members at Harvard University in 2019. So this technology is quite new. Prime editing uses Cas9 nickase, a reverse transcriptase (RT) enzyme, and prime editing guide RNA (pegRNA).
So how does the technology work?
The guide sequence of the pegRNA finds the specific target sequence, then the Cas nickase nicks the DNA. This opens up a single DNA strand, and then reverse transcription using the RT template of the pegRNA can commence. Two flaps form, containing the original strand and the edited strand. The original strand is cleaved by enzymes and the edited strand is stuck to the original DNA sequence. As the DNA now contains an original strand and the new edited strand, the DNA bases are mismatched, therefore, mismatch repair occurs, which either keeps the edited bases or removes them.
Watch the video below to see how prime editing works.
Prime Editing - In almost less than a minute by BoredButResourceful
Where has this technology been used so far?
Prime editing is quite new but it has already been used by some labs. Here are some examples of where prime editing has been used:
David Liu - Sickle cell disease (HBB gene correction)
David Liu - Tay-Sachs disease (HEXA gene correction)
David Liu - Prion disease (PRNP gene)
Qiupeng Lin - In rice and wheat
Haroon Butt - Herbicide resistance in rice
What are the pros and cons of prime editing
Prime editing is an alternative tool to conventional CRISPR gene editing which works by repairing the double-stranded break with NHEJ or HDR a process that can produce indels (which causes issues). Whereas, prime editing works by using DNA mismatch repair.
It has been suggested that prime editing has fewer off-target effects compared to conventional CRISPR gene editing due to having three individual steps to bind to the target DNA.
Gene-based therapies need to reach quite a number of cells in the body in order to mediate a therapeutic effect, therefore, the delivery method used is an important but also limiting factor. A delivery method that is tissue-specific or organ-specific would be required to ensure high efficiency and therapeutic benefit.
Prime editing is a great tool for minor genetic changes, however, the technology may struggle with larger genetic changes as this would need a very large reverse transcription template and this may affect the efficiency.
All in all this technology seems promising from the early research, there is a wide range of CRISPR based tools now, each with its own strength and weakness. Prime editing is quite a versatile tool, more research will only further optimise this tool. Let's see what further research brings!